C (#) sharp - Nullable type (null-coalescing/ double question mark) operator

I came across a very interesting operator today in C#. At first I thought it was like a ternary operator but it turned out not to be. The operator "??", yes two question marks.

It is known as the "nullable type", "null-coalescing" or just the double question mark operator.

I've only come accross it in C# and according to the Microsoft, help page: "The ?? operator returns the left-hand operand if it is not null, or else it returns the right operand."

The remarks went on to say:

"A nullable type can contain a value, or it can be undefined. The ?? operator defines the default value to be returned when a nullable type is assigned to a non-nullable type. If you try to assign a nullable type to a non-nullable type without using the ?? operator, you will generate a compile-time error. If you use a cast, and the nullable type is currently undefined, an InvalidOperationException exception will be thrown."

While its not very common, it can offer very tricky questions in interviews as a friend of mine recently found out. Some example code of using it, (taken from the help page again):

// nullable_type_operator.cs
using System;
class MainClass
{
    static int? GetNullableInt()
    {
        return null;
    }

    static string GetStringValue()
    {
        return null;
    }

    static void Main()
    {
        // ?? operator example.
        int? x = null;

        // y = x, unless x is null, in which case y = -1.
        int y = x ?? -1;

        // Assign i to return value of method, unless
        // return value is null, in which case assign
        // default value of int to i.
        int i = GetNullableInt() ?? default(int);

        string s = GetStringValue();
        // ?? also works with reference types.
        // Display contents of s, unless s is null,
        // in which case display "Unspecified".
        Console.WriteLine(s ?? "Unspecified");
    }
}

zcourts